In the manufacture of liquid-insulated, electric transformers, it is the usual practice to arrange gaps for the passage of an electrically-insulating liquid, for example oil, between parts having different potentials, such as between a winding and an iron core or between two windings. In such gaps there are normally provided spaced-apart barriers made from sheets of pressboard, the transformer insulation thus being built up from pressboard sheets with intermediate gaps. The most common method of achieving the necessary distances between the pressboard sheets is to arrange ribs of pressboard between the sheets. If the barriers are cylindrical, such as when they are arranged between two windings or between a winding and an iron core located inside the winding, the ribs are arranged parallel to the axis of the barriers. The ribs may be glued or tied, for example with tape, to a sheet of pressboard with which they make contact. Between the ribs there are formed spaces through which, during operation of the transformer, the oil or other insulating liquid is caused to pass for cooling the transformer.
During assembly of such a transformer, the creation of plays between the ribs and adjacent pressboard sheets cannot be avoided. When the transformer is subsequently dried out, by suitable heating, prior to the introduction of transformer oil or other insulating liquid, the insulating materials shrink, resulting in the creation of further small plays between the ribs and adjacent pressboard sheets and enlargement of the already existing plays.
In the event of a short-circuit arising in such a transformer, radially and axially directed forces occur in the windings. In a winding which closely surrounds a core leg, the radial forces are directed inwardly towards the core leg. These forces thus tend to reduce the diameter of the winding, and if the winding is not sufficiently resistant or well stayed, it will break or its diameter will be reduced. In a winding which is located further away from the core leg, the forces are in a direction away from the core leg. These forces tend to increase the diameter of such a winding, which may lead to damage, for example displacement of its terminals.
One aim of the present invention is to provide a wedging device which makes possible a considerably improved staying of an electric transformer and, therefore, a considerably improved resistance to damage caused by short-circuit forces. To this end the invention provides a wedging device which, after having been mounted in its intended position, may, without further handling, be caused to increase its dimensions in a desired direction. If wedging devices of this kind are used instead of the above-described ribs, or instead of some of these ribs, in the manufacture of a transformer, the wedging devices, after assembly of the transformer parts, may be caused to increase their dimensions in the radial direction of the transformer windings. In this way it can be ensured that the windings are securely stayed, with no play, by the wedging devices. However, the wedging device in accordance with the invention is also suitable for purposes other than the staying of transformer windings. For example, it may be used for staying, in an analagous manner, the windings of reactors and for staying or wedging of objects in spaces which are not readily accessible.